Semiconductor packages including package substrates having polymer-derived ceramic cores are described. In an example, a package substrate includes a core layer including a polymer-derived ceramic. The polymer-derived ceramic may include filler particles to control shrinkage and reduce warpage of the core layer during fabrication and use of the package substrate. The core layer may include counterbores or blind holes to embed a contact pad or an electrical interconnect in the core layer. A semiconductor die may be mounted on the package substrate and may be electrically connected to the contact pad or the electrical interconnect.